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EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium
EVS26
Los Angeles, California, May 6-9, 2012
A Software Configurable Battery
Barrie Lawson
Chairman of CHEEVC Ltd, Aberdour, Scotland, [email protected]
Keywords: battery calendar life, charge equalization, dynamic charging, power management, reliability
Abstract
The paper describes how battery performance can be improved and its functionality enhanced by software control
of the individual cells in the battery chain. The project was initially set up to find ways of improving high rate
battery performance by using cyclic redundancy to enable rest periods to be introduced during both charging and
discharging but the solution found gave rise to some surprising results and opportunities.
A switching matrix provides controlled access to individual cells and an extra cell in the battery string provides
redundancy. The redundant cell is bypassed and rested while remaining cells carry the load. The rest periods reduce
the stress on the cells resulting in increased battery lifetime and also charge capacity. Sequential cycling rests each
cell in turn so that all cells in the battery age at the same rate.
As a bonus, this switching matrix can be used to provide major enhancements in the battery functionality, simply
through software, without adding any hardware. The system is particularly suitable for mission critical applications
as well as motor controllers such as those used to power Light Electrical Vehicles (LEVs).
The package of benefits includes increased cycle life, increased capacity, lower cost per cycle, variable coarse and
fine power output directly from the battery, lossless cell balancing during both charging and discharging, more
accurate State of Charge (SOC) estimation, immunity to single cell failures and the possibility to deliver a short
term power boost.
1. Introduction
The rate at which a battery or cell can be charged or
discharged is limited by the rate at which the active
chemicals in the cells can be transformed. Forcing
high currents through the battery results in incomplete
transformation of the active chemicals reducing the
battery’s effective charge capacity and it also causes
unwanted, irreversible chemical reactions to occur
because the chemical transformations cannot keep up
with the current demands [1]. The unwanted chemical
transformations consume some of the active
chemicals causing the battery to lose capacity and
thus age prematurely. Examples of this performance
degradation are shown in the following section with
an explanation of the causes. The object of the
CHEEVC design was to find a way of reducing this
inherent performance degradation at high energy
throughput rates.
Linden and Reddy [2] pointed out that introducing
EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium
rest periods during charging to allow for stabilisation
increased the charge capacity and lifetime of alkaline
batteries. Chiasserini and Rao [3] and Mischie and
Toma [4] showed that introducing rest periods during
discharging can increase the available capacity of
Lead acid batteries. Though these studies were both
carried out on alkaline and Lead acid batteries, the
principles involved apply to the process of chemical
transformation and should apply to a greater or lesser
extent to any cell chemistry. Tests were carried out on
Lithium Ion cells to prove that this was the case.
1.1. Capacity / Rate Interdependence
It is well known that increasing the rate at which a
battery operates results in a reduction in capacity and
conversely, reducing the rate of current draw results in
increased capacity. The graph Fig. 1 below which
shows discharge curves for an 8 Ah cell is typical for
Lithium Ion cell chemistry and demonstrates this
effect.
Figure 1:
The capacity reduction at high discharge rates occurs
because the transformation of the active chemicals
cannot keep pace with the current drawn. The result is
incomplete chemical reactions and an associated
reduction in capacity. This may be accompanied by
changes in the morphology of the electrode crystals
such as cracking or crystal growth which adversely
affect the internal impedance of the cell. Similar
problems occur during charging. There is a limitation
as to how quickly the Lithium ions can enter into the
intercalation layers of the anode. Trying to force too
much current through the battery during the charging
process results in surplus ions being deposited on the
anode in the form of Lithium metal. Known as
Lithium plating, this results in an irreversible capacity
loss. At the same time, maintaining the higher
voltages needed for fast charging can lead to
breakdown of the electrolyte which also results in
capacity loss.
1.2. Cycle Life Dependence on Rate
From the above we can expect that with each
charge/discharge cycle the accumulated irreversible
capacity loss will increase. Although this may be
imperceptible, ultimately the capacity reduction will
result in the cell being unable to store the energy
required by the specification. In other words it reaches
the end of its useful life and since the capacity loss is
brought on by high current operation, we can expect
that he cycle life of the cell will be shorter, the higher
the current it carries. The graph Fig.2 by Choi et al [5]
demonstrates that this is the case in practice.
Figure 2:
1.3. Chemical Transformation Rates and
Rest Periods
As already noted, the rate at which current can be
pushed into or drawn from a cell depends on the rate
at which the charging or discharging chemical
EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium
transformations take place. Chemical reactions
cannot take place instantaneously. There are different
reaction gradients between the ions close to the
electrodes and those further away and also within the
electrodes themselves due to the time it takes for the
ions to progress through the cell and there are three
distinct time constants associated with these ion flows
[6]. These are shown in the diagram Fig.3 and
explained below.
Figure 3: Source: Electropaedia
1 Charge transfer / Chemical conversion at the
electrode surface. Chemical conversion of the ions
close to the surface of the electrodes occurs very
quickly and this reaction has a short time constant.
2 Mass transfer / Diffusion of ions through the
electrolyte bulk. Diffusion of ions through the bulk
of the electrolyte is relatively slow and continues until
all materials have been transformed or transferred. It
thus has a long time constant.
3 Intercalation of ions in the electrode bulk.
Diffusion of the ions between the intercalation layers
of the electrode crystals is likewise very slow and so
also has a long time constant.
The maximum current, consistent with complete
transformation of the active chemicals, which can
pass through a cell is thus limited by the rate of the
ion flow through the cells and the reaction time at the
surface of the electrodes. Longer reaction times are
more noticeable in high capacity cells which tend to
be physically very large. High pulse currents may be
possible, accommodated by the ions close to the
electrode surface but continuous currents will be
limited to a lower value due to the longer time it takes
the ions to reach the active electrode crystals and to
percolate through the crystal lattice. The introduction
of “rest periods” during charging and discharging
allows more time for the chemical transformations to
take place and for the reaction to stabilise thus
avoiding the problems associated with incomplete
transformation of the active chemicals.
Reaction rates are of course also temperature
dependent and the maximum current flow is also
limited by the rate of heat dissipation in the cell.
2. Benefits of Rest Periods
Rest periods provide time for completion of the ion
transportation and the chemical reactions to stabilise
(Also known as quiescent recovery). They allow
recovery time during both charging and discharging
thus reducing the stress on the cells, increasing charge
capacity and avoiding unwanted chemical reactions
allowing increased cycle life. They also allow more
accurate SOC estimation.
3. Proof of Concept
While the problems of ion transportation and
transformation apply in principle to all battery
chemistries; and despite the fact that it has been
known for some time battery performance may be
enhanced by the use of rest periods, nobody has
applied it in a controlled way to increase the cycle life
of the cells. We need to know whether this technique
can be implemented in practical batteries and whether
it will be cost effective. For this, two experiments
were set up, the first to determine the potential cycle
life improvement and the second to determine the
Coulombic efficiency as well as to determine the
EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium
optimum duration of the rest periods and to confirm
the application in a working battery.
Tests were carried out on individual cells as well as
on complete packs designed with 1 for 7 redundancy.
That is 7 active cells and 1 stand-by cell.
3.1. Test Method
In all experiments, the industry standard method of
battery cycling was used to charge and discharge
sample cells or complete batteries according to the
manufacturer’s specifications. The cycling equipment
shown in Fig 4 below incorporated programmable
power supplies to charge the cells using a constant
current-constant voltage
charging profile and loads
to discharge the cells. Cell
voltage was continuously
monitored and the cell
capacity was determined
by Coulomb counting.
(Integrating the current
over time).
Figure 4: Cycle Tester
The chart Fig. 5 below is a printout from the cycling
equipment showing cell terminal voltages and
currents during a typical charge/discharge cycle.
Figure 5: Test Charge - Discharge Cycle
Note that the duration of the charge is around 4 hours
and the discharge period is about 2.25 hours.
Following the cell manufacturer’s specification there
is a rest period of 30 minutes at the end of each charge
and another at the end of each discharge period when
the cells are disconnected from the power supply or
the load. This is to allow the battery to approach
equilibrium and to cool down so that each cycle
effectively has a fresh charge avoiding the
accumulation of partially transformed chemicals and
the build up of temperature. This long recovery period
applied to both the tests with the programmed rest
periods and also to the control group without the
programmed rest periods.
For the test, the intention was to rest the cells for short
periods during both the charging and discharging
periods. Rest periods ranged from 0.1 seconds every
0.8 seconds to 10 seconds every 80 seconds.
Unfortunately, because of equipment malfunction, it
was only possible to apply the rest periods during
discharging. (Life tests being repeated on new
equipment. Early indications confirm expectations)
3.2. Cycle Life Testing
Tests were performed with 18650 Lithium ion cells to
verify and quantify the expected cycle life
improvement made possible by the introduction of
rest periods. A batch of 1000 X 18650 cells with a
specified capacity of 2,400 mAh and cycle life of 300
cycles was purchased directly from a well known
manufacturer. Two sample batches of 48 cells were
selected randomly from the 1000 cells, one group for
cycling with rest periods, the other, a control group,
for cycling in the normal way without rest periods.
The cells were mounted in two identical, cycle test
machines each with 48 position test racks standing
side by side in the test room and cycled at room
temperature for 300 complete charge–discharge cycles
with a charge rate of 0.5C and a discharge rate of
1.0C.The capacity of each cell was measured after the
first cycle to establish a baseline and also after 300
cycles to determine the effect of the rest periods over
the specified life time of the cells.
EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium
Figure 6: Increase in Cycle Life Due to Rest Periods
Histograms showing the number of cells with each
level of capacity are shown in Fig.6 above.
One surprising result was the spread of capacity at the
actual beginning of life (BOL) of the new cells and
the shape of the distribution, shown in the top 2
histograms. The cell manufacturer gave assurances
that this was normal so it was discounted for the
purposes of these tests.
The bottom 2 histograms show that under normal
operating conditions there is a wide spread in the
capacity of the cells at the specified end of life (EOL)
of 300 cycles and only 44% of the cells are within the
manufacturer’s specification of 1750 mAh capacity.
The cells which had been cycled with rest periods
demonstrated a much lower spread in capacity and
79% of all cells exceeded the manufacturer’s
specification, an improvement of over 20% in average
lifetime.
The graphs show that by applying rest periods during
discharge alone, the cycle life of the cells can be
increased by 20%. We can expect a similar
improvement by introducing rest periods during
charging, since the principle involved is the same
however as noted above, further tests must be carried
out to confirm this.
3.3. Coulombic Efficiency
The second test had two objectives:
1) To measure the Coulombic efficiency of a battery
pack resulting from the introduction of rest periods.
2) To determine the optimum duration of the rest
periods.
Although the Coulombic (round trip) charge /
discharge efficiency of a Lithium ion cell is close to
100% (Actually 99.86% for the cells used in these
tests), when the cells are incorporated into a battery
pack, the energy consumed by the associated battery
management system (BMS) and the cell balancing
circuits can affect this significantly. For a typical low
power battery using eight 2400 mAh 18650 cells, the
BMS and cell balancing could consume as much as 80
mA during operation [7]. This amounts to an
efficiency loss of 3.3%. These system losses are rarely
EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium
quoted in battery specifications. The CHEEVC design
is intended to work in conjunction with conventional
BMS designs, but with lossless cell balancing
replacing the dissipative cell balancing. See more
about cell balancing in paragraphs 5.1 and 5.2 below.
3.3.1. Tests
The Coulombic efficiency of an 8 cell battery pack
was determined, during three complete charge /
discharge cycles, with and without rest periods by
measuring the charge applied during charging and the
stored capacity by discharging the battery. The
charging profile and the charge discharge rates were
the same as in the previous tests.
3.3.2. Results
The graph Fig 7 below shows the Coulombic
efficiency of an 8 cell pack, including BMS, charged
and discharged with different rest periods. A
switching matrix (See paragraph 4) was used to
implement the cycles with rest periods but not for the
reference cycle.
Figure 7: Coulombic Efficiency of an 8 Cell Battery
Pack With and Without Rest Periods.
It shows an increase in Coulombic efficiency due to
the rest periods of around 0. 35%. This is in line with
the increase in charge capacity predicted by the
Peukert relationship. C = I n
T where "C" is the
theoretical (specified) capacity of the battery
expressed in Amphours, "I" is the current, "T" is time,
and "n" is the Peukert Number, a constant for a given
battery. For Lithium cells “n” is typically 1.03 or less
and this would result in a capacity improvement of
0.4% with a rest period ratio of 1 for 7. The graph also
indicates that there is no further improvement by
increasing the rest periods from 0.1 second to up to 90
seconds. It also shows that the overall Coulombic
efficiency loss is not increased, from the expected 3%
due to the BMS, by the addition of the switching
matrix. This is partly due to the small improvement
due to the Peukert effect, but mainly because the
dissipative cell balancing is replaced with lossless
balancing.
4. Stress Reduction by Cyclic
Redundancy
While the introduction of rest periods can improve
battery performance, the cost, or trade off, is
intermittent energy flow which is unacceptable for
most applications. Cyclic redundancy avoids this
problem by enabling individual cells to be taken out
of the battery chain and rested while at the same time
a constant current into or out of the battery is
maintained. In effect this means that the battery can
operate at constant full power with the maximum
instantaneous current that the cells are specified to
carry, while the each of the cells which make up the
battery actually carries a lower average current due to
the effect of the rest periods. Key to the design is the
provision of a stand-by or redundant cell and the
switching matrix which allows each individual cell in
the chain to be rested by switching it out of the battery
chain in sequence to take its turn as the stand-by cell.
The availability of the stand-by cell to replace a failed
cell provides a major improvement in battery
reliability. More importantly the introduction of rest
periods for the cells during both charging and
discharging reduces the stress on the battery during its
use and this in turn brings improved cycle life as well
96.38% 96.59% 96.87% 96.92% 97.08% 96.63%
97.04% 96.84%
90.00%
92.00%
94.00%
96.00%
98.00%
100.00%
0.0s 0.1s 0.5s 1s 10s 30s 60s 90s
Effi
cie
ncy
%
Rest Period Duration (Seconds)
Battery Pack Coulombic Efficiency
EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium
as increased capacity of the battery. An alternative
way of looking at this is to consider that the
incorporation of rest periods is simply a method of
reducing the average current through the cells which
in turn reduces the stress on the cells. The provision of
the redundant cell is a way of maintaining the full
power output of the battery pack.
4.1. Implementation
CHEEVC’s solution is to provide the necessary rest
periods by means of cyclic redundancy. An extra
“stand-by” cell in the battery string provides
redundancy. The redundant cell is bypassed and
rested while the remaining cells carry the load.
Sequential cycling rests each cell in turn. All cells in
the battery age at the same rate. Resting takes place
during both charge and discharge cycles.
The diagram Fig. 8 below shows the principle of
cyclic redundancy. Each sell is bypassed
sequentially under software control using 2 or more
switches, in this case Field Effect Transistors
(FETs), in a switching matrix.
4.2. Switching Matrix Functions
The switching matrix provides access to, and control
of, individual cells in the battery chain. It provides
rest periods by the sequential bypassing of individual
cells and it allows software configurable features
without adding any additional hardware. In the
example Fig.8 below, one redundant cell is provided
for seven active cells providing 1 for 7 redundancy.
The switching circuit causes each cell to be
sequentially cycled out of the active cell string thus
allowing a rest period for each cell every 8 cycles.
5. Additional Benefits of Cell Control
With minor additions, the switching matrix can be
used to provide cell balancing or for variable power
control. Since multiple cells may need to be bypassed,
an extra switch in each bypass path, that is three
switches per cell, may be needed as in Fig. 9 below.
5.1. Lossless Cell Balancing
Cell balancing is designed to equalise the stress on all
the cells is a battery chain to avoid overstressing the
weaker cells which could cause premature failure of
the battery. Conventional passive cell balancing uses a
switch to divert the excess charge from over-charged
cells through a bleed or bypass resistor which
dissipates the excess charge.
The switching matrix can be used to provide lossless
cell balancing as shown in the circuit below. It works
during charging by increasing the rest periods of cells
with a higher SOC until the others catch up thus
avoiding overcharge, and during discharging by
increasing the rest periods of cells with a lower SOC.
Since there is no excess charge to dissipate, the circuit
is lossless.
Figure 8: The Switching Matrix
EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium
Figure 9: Switching Matrix with 3 Switches per Cell
5.2. Dynamic Equalisation
A second drawback of conventional cell balancing is
that it is usually only possible to carry it out when the
battery is fully charged towards the end of its
discharge cycle and only when the battery is not in
use. Bypass currents are very low so that equalisation
is a very slow process which may take many hours to
complete. The lossless cell balancing described above
is able to keep all cells at the same state of charge
(SOC) at any level during both charging and
discharging so that cells are balanced at all times
during operation and not just when the battery is idle.
Preventing cells from getting out of balance prolongs
battery life even further.
5.3. Variable Power Output
The ability to bypass multiple cells can be used to
provide a variable power output directly from the
battery. This facility can be used in some applications
to avoid the costs and inefficiencies of separate power
controllers. It is also useful as a soft start feature to
control inrush currents in power circuits.
5.3.1. Variable Output Voltage
It is possible to use the switching matrix to
provide coarse voltage control by varying the
number of active cells in the battery chain simply
by bypassing multiple cells.
5.3.2. Variable Current Output
The same circuit can also be used to provide fine
power control by using one or more of the
existing switches to interrupt the current in the
main current path through the battery for short
controlled intervals in much the same way as
pulse width modulation (PWM) works. Some
smoothing of the output current may also be
necessary depending on the application.
5.4. Fail Safe Battery Isolation
The switching matrix allows individual cells or the
complete battery to be isolated in case of an
emergency.
5.5. More Accurate State of Charge
(SOC) Estimation
The accuracy of the SOC estimation is important in all
EV applications. Like the gas gauge in a car, it
provides an indication of the remaining range
available from the energy left in the battery. In HEVs
it may also be used to determine when the engine is
switched on and off and so any errors could affect the
vehicle’s fuel efficiency.
SOC cannot be measured directly and for Lithium
batteries it is usually calculated from Coulomb
counting modified by an algorithm which takes into
account the battery or cell voltage, the cell chemistry
variant, the battery temperature, the rate of current
draw and possibly other factors. But in all cases a
major factor affecting the SOC calculation is the
battery or cell voltage.
The graph Fig. 10 below, often called the hysteresis
curve of the battery, shows that the actual measured
cell voltage depends on whether it was taken when the
battery was being charged or discharged. This is
because the measurement is affected by the external
circuitry and the voltage drop across the battery as
well as the time lag in reaching chemical
equilibrium consistent with the current flow as
discussed in paragraph 1.3.
Figure 10: SOC Hysteresis Curve
There can only be one state of charge, so the deviation
from the central quiescent voltage line can introduce
EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium
large inaccuracies. If the charging or discharging were
stopped completely at any time during the cycle the
cell voltage would migrate towards the quiescent
central position between the charge and discharge
curves as the chemical transformations stabilise.
The graph also shows that when rest periods are
introduced the measured cell voltage moves towards
its quiescent condition during the rest periods. This is
because the cell is in the open circuit condition during
the rest period and the chemical transformations in the
cell have time to complete. Thus, the cell voltage
measured towards the end of the rest period will be
much closer to the cell voltage corresponding to the
true open circuit voltage of the cell and the
measurement error introduced into the SOC
calculation will be minimised.
7. Additional Benefits of Cyclic
Redundancy
The availability of the standby cell needed for
cyclic redundancy brings additional opportunities
and benefits to the battery.
7.1. Increased Available Capacity
The cyclic redundancy allows the full capacity of the
extra cell to be used by the battery. With 1 for 7
redundancy this amounts to an extra increase in
capacity of 14.3%. Thus the standby cell is fully
occupied and does not reduce the battery energy
density. As an alternative, smaller capacity cells could
be used to provide a given battery capacity.
7.2. .Increased Reliability
The extra, stand-by cell can be used in the
conventional way to provide immunity to single cell
failures, increasing the system reliability by bypassing
or replacing a failed cell. In this case the cyclic
redundancy would no longer be available but the
battery would continue to function at full power until
it could be serviced. This can be very important in
mission critical and airborne applications.
In a multi cell battery with n cells in a series chain
each with a reliability or survivability of R% per year.
If the failure of any cell will cause the failure of the
battery, then the reliability of the battery will be Rn.
The more cells in the chain the less reliable the battery
will be. For a battery constructed from 7 series cells
each with a reliability of 99.99% per year or 1 failure
in 10,000 batteries, the reliability of the battery will be
99.93% per year or 1 failure in 1,429 batteries. With 8
cells it will be 99.92% per year or 1 in 1,250 batteries.
Reliability with redundancy is given by
Where k is the minimum number of components
needed to keep the system functioning [7]. As before
with R=99.99% and n=8 and k=7 in a 1 for 7 system
R is calculated to be 99.99994% per year or 1 failure
in 1.67 million batteries. A dramatic improvement.
7.3. Time Limited Power Boost
The standby cell can also be switched in to the battery
chain to provide a time limited power boost of 30.6%
in a system with 1 for 7 redundancy. However this
will disable the cyclic redundancy and should
probably only be used in an emergency.
EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium
8 Alternative Cell De-stressing
Method
Providing rest periods is essentially a way of de-
stressing the cells or the battery. There are other
alternative ways of accomplishing this. Fig 11 by
Choi [5] Shows how cycle life can be increased by
reducing the charge level on the cells.
Figure 11: Cycle life and Charge Level
But there are drawbacks to this solution. The number
of cells must be increased in proportion to make up
for the decreased cell capacity resulting from the
lower charge level. The battery energy density is
reduced since the cells are not used at their full
capacity. Reliability is reduced because there will be
more cells in the chain and a single cell failure will
cause failure of the battery.
9 Practical Systems
The CHEEVC switching matrix works in conjunction
with a conventional Battery Management System
(BMS) to provide extra functionality and enhanced
benefits to the battery. The overall system block
diagram is shown in Fig 12 below. Cost savings can
be made by accommodating the software needed to
operate the switching matrix in the same
microprocessor used by the BMS
10 Scalability
Though the initial applications are for electric bikes,
the design can be scaled up in 2 ways to cater for high
power applications. Higher power FET switches can
be used. As the power requirement increases, the cost
of the cells increases more quickly than the cost of the
FETs improving the cost effectiveness of the design.
However the downside is that such applications will
require more sophisticated thermal management to
cope with the increased heat dissipation of the higher
power cells and the associated electronic circuits. By
using higher power FETs current handling capacities
of 400 amps are possible.
The alternative method of increasing the power
handling capacity of the battery is to build up the
battery from low capacity modules as is done in the
Tesla car.
11 Summary of System Features,
Functions and Benefits
The histogram Fig 13 below shows a summary of the
benefits compared with the alternative de-stressing
method of reducing the cell voltage.
Extended battery life of over 20%. Achieved by the
use of a switching matrix to provide rest periods
during both charging and discharging. It uses an extra
cell to enable cyclic redundancy of the individual cells
in the battery chain. The use of rest periods in this
way is unique technology for which a patent
application was filed in November 2010. (A 20%
improvement was demonstrated by applying rest
periods during discharging only. 30% or more is
possible when rest periods also applied during
charging.)
EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium
Figure 12: System Block Diagram
Reduced operating costs of 5% per cycle during
the lifetime of the battery. A direct consequence of
the extended life from the cells. (Taking into account
the increased cost of the battery)
Software controlled power output. Without any
additional hardware, the switching matrix can be
programmed to provide a variable voltage and current
from the battery. No other battery has such a
capability. This is also a unique technology which is
also covered by the above noted patent. This facility
can be used to replace the power control function
normally required in motor control applications thus
eliminating the need for bulky, costly and inefficient
DC-DC converters or lossy rheostats.
Immunity from single cell failures. The cyclic
redundancy provided by the switching matrix
dramatically improves reliability. This is very useful
for mission critical applications. In electric vehicles
(or aviation applications) it will always get you home.
Figure 13: A Graphical Summary of the Benefits
EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium
Improved accuracy of State of Charge (SOC)
estimation. The cell voltage, which is one of the main
parameters from which the SOC is derived, can be
measured when the cell is in the open circuit condition
and the chemical transformation in the cell has
stabilised at the end of the rest period. This results in a
more accurate measurement.
Increased battery capacity. The capacity of the extra
cell in the chain, the so called “redundant cell” is
actually used to increase the capacity of the battery.
Short term power boost As above, the redundant cell
can also be used to provide an emergency power boost
of 30% by switching off the cyclic redundancy and
using the power of all of the cells in the chain. The
cyclic redundancy is of course switched off during
this boost.
Works with any cell chemistry. While this is
technically possible. Only applications using various
variants of Lithium Ion chemistry are considered.
Does not require a special charger Works with any
standard charger designed for Lithium batteries.
Does not depend on technology breakthroughs. The
battery can be realised using conventional hardware
and software technology
Costs and Trade-offs
The cost to provide all of these benefits is an increase
of about 14% in the cost of the battery.
12 Applications
In principle, the technology can be used with any
multi-cell battery of which billions are sold every
year. The first generation products will be suitable for
applications with power levels up to about 1kW and
capacities up to about 2 kWh. This includes batteries
used to power Light Electric Vehicles (LEVs) such as
electric bikes, scooters and mobility aids, high
capacity power tools, airborne surveillance drones,
remote control models and motor control applications.
Second generation products will be designed for
higher power applications such as electric and hybrid
electric vehicles (EVs and HEVs).
13. References
[1] The Electropaedia
http://www.mpoweruk.com/life.htm Accessed 20/Jan
/2012
[2] David Linden & Thomas B. Reddy: Pulse
Charging , Handbook of batteries 3rd
edition, Section
36.5.3
[3] Carla F. Chiasserini & Ramesh R. Rao: A Model
for Battery Pulsed Discharge with Recovery Effect,
WCNC'99, New Orleans, USA, September 1999
[4] S. Mischie & L. Toma: WSEAS Transactions on
Power Systems, Issue 3, Volume 3, March 2008
[5] Soo Seok Choi & Hong S Lim - Journal of Power
Sources, Volume 111, Issue 1, 18 September 2002
[6] The Electropaedia
http://www.mpoweruk.com/chargers.htm Accessed
20/Jan /2012
[7] ReliaSoft Corporation.
http://www.weibull.com/SystemRelWeb/k-out-of-
n_parallel_configuration.htm Accessed 20/Jan /2012
Author
Barrie Lawson graduated with a degree in Electrical and
Electronic Engineering from Birmingham University in
1964. He is Chairman of CHEEVC, a battery R&D
company in Scotland. In his spare time he is the author of
the Electropaedia at www.electropaedia.com , a web site
about battery and energy
technologies.